Flight Simulation and Control of a Helicopter Undergoing Rotor Span Morphing

The flight mechanics, stability, and control of a helicopter undergoing rotor span morphing is examined. A span-morphing variant of the UH-60A Black Hawk helicopter at 18,300 lb gross weight is developed. A model-following dynamic inversion controller is implemented, and radius change is introduced as an additional feed-forward component into the control laws, with the aircraft model being updated with respect to flight condition. The flapping and inflow modes of the rotor exhibit significant movement as span changes, but relatively little movement is observed in the rigid-body modes. Closed-loop poles associated with the low-frequency aircraft modes are observed to be robust to change in rotor span, eliminating the need for model updates during the morphing process. The error compensators in the control laws use proportional-integral-derivative control for roll and pitch attitude and proportional-integral (PI) control for lateral and longitudinal ground speed, but require proportional-double integral compensator control for vertical speed and yaw rate to avoid altitude loss and heading drift observed with only PI control, during span morphing. Detailed simulations results are presented for 40 kt cruise, and velocities of 80 and 120 kt are also considered. From a baseline rotor radius of 26.8 ft, retraction to 22.8 ft, as well as extension to 31.5 ft, is considered, nominally over a 60-s duration. The controller is observed to regulate the aircraft operating state well over the nominal duration and is still effective when duration is reduced to 30 s.